Hi,
you could modify the default log-in page. The IP address of visitor is available in the request.
In case it is the one you need, you could authenticate the user. Because visitor comes to the site as anonymous, you would need to authenticate one common user which would be used for authentication and everybody from that one PC would use it.
CookieHelper.EnsureResponseCookie(FormsAuthentication.FormsCookieName);
CookieHelper.ChangeCookieExpiration(FormsAuthentication.FormsCookieName, DateTime.Now.AddMinutes(System.Web.HttpContext.Current.Session.Timeout), false);
// Ensure response (authentication) cookie
FormsAuthentication.SetAuthCookie(user.UserName, true);
Best regards,
Ivana Tomanickova